The electric double-layer capacitor has a structure where a capacitor element impregnated with an electrolyte solution is housed in a metal case and open ends thereof are sealed. The capacitor element is formed by winding or laminating polarized electrodes, which is prepared by providing a polarized electrode layer on the surface of a metal foil such as aluminum, with a separator interpositioned in between.
This electric double-layer capacitor attempts improvement of power density by reducing the thickness of the electrode in applications of power supply for motor drive of electric vehicles and hybrid cars, or power application for regenerative energy storage on braking. For this purpose, better thinning can be attempted with a coated electrode prepared by coating a slurry electrode using latex etc. as the binder onto a current collector compared to a sheet electrode prepared by forming an electrode into a sheet using polytetrafluoroethylene etc. as the binder and adhering this onto a current collector.
This coated electrode comprises an electrode material as a base material, a conductive auxiliary agent, a binding agent (binder) for binding the base material and the electrically conductive auxiliary agent, and further, a dispersing agent used for dispersing the base material and the electrically conductive auxiliary agent in water and slurring.
As electrode materials employed as cathodes/anodes, activated carbon, polyacene, or the like can be used. For activated carbons, for example, resin-based carbons such as phenol resins, plant-based carbons such as coconut shell, coal/petroleum pitch cokes, and mesocarbonmicrobeads (MCMB) etc. are activated and employed. For electrically conductive auxiliary agents, ketjen black, acetylene black, and natural/artificial black lead etc. are employed. For dispersing agents, hydroxymethylethylcellulose (HMEC), hydroxypropylmethylcellulose (HPMC), and carboxymethylcellulose-sodium salt (CMC-Na) are employed. For binding agents, it is preferred to use latexes of acrylic elastomers which are water-based binding agent that allow easy handling. Water is used as the solvent for stirring the polarized electrode material and the electrically conductive auxiliary agent.
Furthermore, high capacity and superior long-term reliability is demanded for electric double-layer capacitors, and carbonate solvents such as propylene carbonate or acetonitrile have been used as the electrolyte solution for conventional electric double-layer capacitors. According to this, it is said that an electric double-layer capacitor having high capacity as well as superior high temperature load can be obtained for use at 60° C.
However, in an electrolyte solution that employs carbonate solvent, there is a problem that the internal pressure of the container housing the polarized electrode and electrolyte solution increases due to carbon monoxide (CO) gas generated from the degradation of the solvent under high temperatures. For this reason, 60° C. was the limit, and there was a problem that it cannot deal with use at further higher temperatures of 70-85° C. At the same time, there is an attempt to enable use at 70° C. by employing γ-butyrolactone (See Japanese Patent Application Laid-open No. 2001-217150).
However, when life test at 85° C. is performed on an electric double-layer capacitor having a coated electrode that employs an electrolyte solution comprising triethylmethylammoniumtetrafluoroborate (TEMABF4) and γ-butyrolactone (GBL) as the solvent, activated carbon as the base material, ketjen black as the electrically conductive auxiliary agent, and acrylic elastomer (e.g. acrylnitrile rubber) as the binding agent, it was found that increase in the internal resistance (DCIR) of the capacitor was large.
In other words, as shown in FIG. 4, loading experiments spanning 500 hours at each temperatures of 60° C., 70° C., and 85° C. were performed for each of the coated electrode and the sheet electrode to measure the change in capacity and internal resistance, and it was found that only the internal resistance of the coated electrode at 85° C. was significantly deteriorated in property.
From this knowledge, that is, paying attention to the difference in the expansion rates of binders for the coated electrode and the sheet electrode in solvent, the expansion rates of binding agent ingredients in the electrolyte solution were measured. For the measurement method, as shown in FIG. 5, an acrylic elastomer dissolved in a predetermined solvent was casted onto a plate to form a thin film, and this was dried to produce a binding agent film having a diameter of 20 mm.
This binding agent film was immersed in an electrolyte solution comprising γ-butyrolactoneat each of 20° C., 60° C., 70° C., and 105° C. for 200 hours, and then thickness and diameter thereof were measured to determine the expansion degree of the binding agent film. FIG. 6 is a graph showing the expansion rate of the binding agent film after electrolyte solution immersion. According to this test result, it was found that when using an electrolyte solution comprising γ-butyrolactone (GBL) as the solvent, the expansion degree of the acrylic elastomer in the high temperature region was larger compared to when using an electrolyte solution comprising propylene carbonate (PC) or acetonitrile (AN) as the solvent.
The object of the present invention is to provide an electric double-layer capacitor, in which even when a coated electrode comprising an electrode material and an electrically conductive auxiliary agent bound by a binding agent using water as the solvent is employed with respect to an electrolyte solution comprising γ-butyrolactone, deterioration of property can be prevented even at a temperature range of approximately 85° C.
In addition, the object of the present invention is to provide a current collector for electric double-layer capacitor and an electric double-layer capacitor that prevents deterioration of properties even at a temperature range of approximately 85° C. in an electric double-layer capacitor that employs an electrolyte solution comprising a fluorine-containing anion.